Promise support

[Ellisande]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Feature

What is the current behavior? (You can also link to an open relevant tickets here)

Promise mocks are not supported

What is the new behavior (if this is a feature change)?

It is now possible to mock functions that return a promise.

Where should the reviewer start?

Then then object.

This is how I tested it

Lots of unit tests.

Does this PR introduce a breaking change?

No If yes:

• [ ] Major version changed

  Please check if the PR fulfills these requirements**

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)
• [x] Version changed by at least patch version

[Ellisande]

@guyellis if you get a chance could you just look over the README and let me know if you think it reads well? I might have been tired and inebriated while writing it so an extra set of eyes would be a big help =) https://github.com/Ellisande/mockolate/blob/promise-support/README.md

[Ellisande]

Closes #6